Spring Boot 文件上传原理解析

Spring Boot 文件上传原理解析

首先我们要知道什么是Spring Boot，这里简单说一下，Spring Boot可以看作是一个框架中的框架--->集成了各种框架，像security、jpa、data、cloud等等，它无须关心配置可以快速启动开发，有兴趣可以了解下自动化配置实现原理,本质上是 spring 4.0的条件化配置实现，深抛下注解，就会看到了。

说Spring Boot 文件上传原理 其实就是Spring MVC，因为这部分工作是Spring MVC做的而不是Spring Boot，那么，SpringMVC又是怎么处理文件上传这个过程的呢？

图:

首先项目启动相关配置，再执行上述第二步的时候 DispatcherServlet会去查找id为multipartResolver的Bean，在配置中看到Bean指向的是CommonsMultipartResolve，其中实现了MultipartResolver接口。

第四步骤这里会判断是否multipart文件即isMultipart方法，返回true：就会调用 multipartResolver 方法，传递HttpServletReTBPeuquest会返回一个MultipartHttpServletRequest对象，再有DispatcherServlet进行处理到Controller层；返回false：会忽略掉，继续传递HttpServletRequest。

在MVC中需要在配置文件webApplicationContext.xml中配置 如下：

而Spring Boot已经自动配置好，直接用就行，做个test没什么问题。有默认的上传限制大小，不过在实际开发中我们还是做一些配置的，

如下在application.properties中：

# multipart config

#默认支持文件上传

spring.http.multipart.enabled=true

#文件上传目录

spring.http.multipart.location=/tmp/xunwu/images/

#最大支持文件大小

spring.http.multipart.max-file-size=4Mb

#最大支持请求大小

spring.http.multipart.max-request-size=20MB

当然也可以写配置类来实现，具体的就不做展示了。

看完上述你肯定有个大概的了解了，这里再啰嗦下，Spring提供Multipart的解析器：MultipartResolver，上述说的是CommonsMultipartResolver，它是基于Commons File Upload第三方来实现，这也是在Servlet3.0之前的东西，3.0+之后也可以不需要依赖第三方库，可以用StandardServletMultipartResolver，同样也是实现了MultipartResolver接口，我们可以看下它的实现：

* Copyright 2002-2017 the original author or authors.

package org.springframework.web.multipart.support;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.Part;

import org.apache.commons.logging.LogFactory;

import org.springframework.web.multipart.MultipartException;

import org.springframework.web.multipart.MultipartHttpServletRequest;

import org.springframework.web.multipart.MultipartResolver;

/**

* Standard implementation of the {@link MultipartResolver} interface,

* based on the Servlet 3.0 {@link javax.servlet.http.Part} API.

* To be added as "multipartResolver" bean to a Spring DispatcherServlet context,

* without any extra configuration at the bean level (see below).

*

*

Note: In order to use Servlet 3.0 based multipart parsing,

* you need to mark the affected servlet with a "multipart-config" section in

* {@code web.xml}, or with a {@link javax.servlet.MultipartConfigElement}

* in programmatic servlet registration, or (in case of a custom servlet class)

* possibly with a {@link javax.servlet.annotation.MultipartConfig} annotation

* on your servlet class. Configuration settings such as maximum sizes or

* storage locations need to be applied at that servlet registration level;

* Servlet 3.0 does not allow for them to be set at the MultipartResolver level.

*

* @author Juergen Hoeller

* @since 3.1

* @see #setResolveLazily

* @see HttpServletRequest#getParts()

* @see org.springframework.web.multipart.commons.CommonsMultipartResolver

*/

public class StandardServletMultipartResolver implements MultipartResolver {

private boolean resolveLazily = false;

/**

* Set whether to resolve the multipart request lazily at the time of

* file or parameter access.

*

Default is "false", resolving the multipart elements immediately, throwing

* corresponding exceptions at the time of the {@link #resolveMultipart} call.

* Switch this to "true" for lazy multipart parsing, throwing parse exceptions

* once the application attempts to obtain multipart files or parameters.

*/

public void setResolveLazily(boolean resolveLazily) {

this.resolveLazily = resolveLazily;

}

@Override

public boolean isMultipart(HttpServletRequest request) {

// Same check as in Commons FileUpload...

if (!"post".equals(request.getMethod().toLowerCase())) {

return false;

}

String contentType = request.getContentType();

return (contentType != null && contentType.toLowerCase().startsWith("multipart/"));

}

@Override

public MultipartHttpServletRequest resolveMultipart(HttpServletRequest request) throws MultipartException {

return new StandardMultipartHttpServletRequest(request, this.resolveLazily);

}

@Override

public void cleanupMultipart(MultipartHttpServletRequest request) {

// To be on the safe side: explicitly delete the parts,

// but only actual file parts (for Resin compatibility)

try {

for (Part part : request.getParts()) {

if (request.getFile(part.getName()) != null) {

part.delete();

}

}

}

catch (Throwable ex) {

LogFactory.getLog(getClass()).warn("Failed to perform cleanup of multipart items", ex);

}

}

}

这里是之前写的test的后者实现配置类，可以简单看下，作为了解：

package com.bj.config;

import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;

import org.springframework.boot.autoconfigure.web.MultipartProperties;

import org.springframework.boot.context.properties.EnableConfigurationProperties;

import org.springframework.context.annotation.Bean;

import org.springframework.context.annotation.Configuration;

import org.springframework.web.multipart.MultipartResolver;

import org.springframework.web.multipart.support.StandardServletMultipartResolver;

import org.springframework.web.servlet.DispatcherServlet;

import javax.servlet.MultipartConfigElement;

@Configuration

@EnableConfigurationProperties(MultipartProperties.class)

public class FileUploadConfig {

private final MultipartProperties multipartProperties;

public FileUploadConfig(MultipartProperties multipartProperties){

this.multipartProperties=multipartProperties;

}

/**

* 注册解析器

* @return

*/

@Bean(name= DispatcherServlet.MULTIPART_RESOLVER_BEAN_NAME)

@ConditionalOnMissingBean(MultipartResolver.class)

public StandardServletMultipartResolver multipartResolver(){

StandardServletMultipartResolver multipartResolver = new StandardServletMultipartResolver();

multipartResolver.setResolveLazily(multipartProperties.isResolveLazily());

return multipartResolver;

}

/**

* 上传配置

* @return

*/

@Bean

@ConditionalOnMissingBean

public MultipartConfigElement multipartConfigElement(){

return this.multipartProperties.createMultipartConfig();

}

}

总结

以上所述是给大家介绍的Spring Boot 文件上传原理解析，希望对大家有所帮助，如果大家有任何疑问请给我留言，会及时回复大家的。在此也非常感谢大家对我们网站的支持！

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